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Python + Kivy (second screen is loading blank)

I am learning kivy now... I am developing my 1st app for a friend, a very simple one. But I am facing this error:

Whenever I click "create account", the named 'Login(Screen)' loads blank. None of the widgets that I have created on my kivy file shows.

here are the codes:

==========================================================================

python file:

from kivy.app import App

from kivy.uix.screenmanager import Screen, ScreenManager


class Gerenciador(ScreenManager):
pass

class BoasVindas(Screen):
pass

class Login(Screen):
def logar(self, usuario, senha):
print("usuario=0, senha=1".format(usuario, senha))

class Resultado(Screen):
pass

class LoginApp(App):

def build(self):
return Gerenciador()

LoginApp().run()


========================================================================

kivy file:

<Gerenciador>:
BoasVindas:
name: 'boasvindas'
BoxLayout:
orientation:'vertical'
Label:
text:'Faça o seu Login ou crie uma nova conta'
Button:
text:'Login'

Button:
text:'Criar nova conta'
on_release:root.current='login'


Login:
name: 'login'
BoxLayout:
TextInput:
id:usuario
hint_text:'Usuário'
multiline: False
TextInput:
id:senha
hint_text:'Senha'
multiline: False
password: True
Button:
id:'btn'
text:'Ok'
on_press: self.parent.parent.logar(usuario.text, senha.text)
on_release:root.current='boasvindas'


=========================================================================

Any ideas on what I am missing? The first screen loads perfectly. If I swap the order, Login screen loads well. But the second screen is blank, no matter what content. as long as it is the second screen to load, it returns blank.

Thank you!

2 Answers
2

In your __init__() method for Gerenciador you are creating the Login and BoasVindas screens, but you are also crating them in your .kv file. You should not do both. If you create them in the .kv file, here is what your code can look like:

__init__()

Gerenciador

Login

BoasVindas

.kv

.kv

python file:

from kivy.app import App

from kivy.uix.screenmanager import Screen, ScreenManager


class Gerenciador(ScreenManager):
pass

class BoasVindas(Screen):
pass

class Login(Screen):
pass

class Resultado(Screen):
pass

class TesteLogin(App):

def build(self):
return Gerenciador()

TesteLogin().run()


kv file:

<Gerenciador>:
BoasVindas:
name: 'boasvindas'
BoxLayout:
orientation:'vertical'
Label:
text:'Faça o seu Login ou crie uma nova conta'
Button:
text:'Login'

Button:
text:'Criar nova conta'
on_release:root.current='login'

Login:
name: 'login'
BoxLayout:
TextInput:
id:usuario
hint_text:'Usuário'
multiline: False
TextInput:
id:senha
hint_text:'Senha'
multiline: False
password: True
Button:
id:'btn'
text:'Ok'
#on_press:self.parent.parent.logar(usuario.text, senha.text)
on_release:root.current='boasvindas'


In your .kv file, when a class name is surrounded by <>, that means that it is a template for building that class, but does not actually create the class. The classes without <> are created, but the ones in your .kv file are subordinate to the Gerenciador class. So, when an instance of the Gerenciador class is created (as in your App.build() method), the Login and BoasVindas screens are created as its children. Note that I added the name attribute to those screens in the .kv file.

.kv

<>

<>

.kv

Gerenciador

Gerenciador

App.build()

Login

BoasVindas

name

.kv

.kv

testelogin.kv

Problem - Second Screen Blank

There were four screens created. Two screens with children widgets and no screen names were created from the kv file. Another two screens with screen names but no children widgets were created from the Python code.

When the button, 'Criar nova conta' was pressed to go to the screen with name, 'login' it was using the instance that was created from the Python code which does not have any widgets in it i.e. it is blank.

'Criar nova conta'

'login'

Build/Processing Sequence

In Kivy App, the order of processing is as follow. To proof this, add print() and id() functions into the kv file, and Python code, __init__() method for each screen. It will show four different screens and memory location for each screen respectively.

id()

__init__()

class Gerenciador(ScreenManager):

def __init__(self):
super(Gerenciador, self).__init__()
print("nGerenciador.__init__():", id(self), "n")
self.add_widget(Login(name='login'))
self.add_widget(BoasVindas(name='boasvindas'))


class BoasVindas(Screen):

def __init__(self, **kwargs):
super(BoasVindas, self).__init__(**kwargs)
print("nBoasVindas.__init__():", id(self), "n")


class Login(Screen):
def __init__(self, **kwargs):
super(Login, self).__init__(**kwargs)
print("nLogin.__init__():", id(self), "n")


class TestLogin(App):

def build(self):
print("nTestLogin.build():")
return Gerenciador()


Button:
text:'Criar nova conta'
on_release:
print("self.parent.parent=", self.parent.parent)
print("id(self.parent.parent)=", id(self.parent.parent))
print("app.root.screen_names=", app.root.screen_names)
print("app.root.screens=", app.root.screens)
for x in app.root.screens: print("screen=0, id(screen)=1".format(x, id(x)))
root.current='login'


Solution

Please refer to the step-by-step solution and example for details.

__init__()

pass

name

name: 'boasvindas'

name: 'login'

self.parent.parent.logar(usuario.text, senha.text)

root.logar(usuario.text, senha.text)

Example

from kivy.app import App
from kivy.uix.screenmanager import Screen, ScreenManager


class Gerenciador(ScreenManager):
pass


class BoasVindas(Screen):
pass


class Login(Screen):

def logar(self, usuario, senha):
print("usuario=0, senha=1".format(usuario, senha))


class Resultado(Screen):
pass


class TesteLogin(App):

def build(self):
return Gerenciador()


TesteLogin().run()


#:kivy 1.11.0

<Gerenciador>:
BoasVindas:
name: 'boasvindas'
BoxLayout:
orientation:'vertical'
Label:
text:'Faça o seu Login ou crie uma nova conta' # Sign in or create a new account
Button:
text:'Login'

Button:
text:'Criar nova conta' # Create new account
on_release:root.current='login'

Login:
name: 'login'
BoxLayout:
TextInput:
id:usuario
hint_text:'Usuário' # User
multiline: False
TextInput:
id:senha
hint_text:'Senha' # Password
multiline: False
password: True
Button:
id:'btn'
text:'Ok'
on_press: self.parent.parent.logar(usuario.text, senha.text)
on_release:root.current='boasvindas' # Welcome


Output

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